I had the dealer throw the cover in for free when I bought the car. I have been using it regularly on my 7-passenger Sorento (with 3rd row down) and never even noticed an interference with the 3rd row seat belts.
In my model, there are two different pairs of slots where you can install it depending on how far back you want to lean the 2nd row seats.
By the way, for when you need to pull the 3rd row seats up and you don't have a chance to leave the cover out of the car (e.g. in the garage), there is a third pair of slots, roughly in the area where the jack compartment is, which work great for me for storing the cover.